A hybrid vehicle is well known that includes an electric differential portion having a differential mechanism distributing power from an engine to a differential electric motor and an output rotating member and an electric motor for running coupled in a power transmittable manner (i.e., coupled directly or indirectly via a gear mechanism) to the output rotating member such that a differential state of the differential mechanism is controlled by controlling an operating state of the differential electric motor. For example, this corresponds to a hybrid vehicle described in Patent Document 1.
In the hybrid vehicle as described above, for example, gear bearing noise may occur from a gear mechanism making up the electric differential portion. Specifically, the gear mechanism has a gap in a meshing portion between mutually meshing gears. For example, if the output torque of the electric motor for running is close to substantially zero, including the value zero [Nm] (i.e., if the electric motor for running is in a floating state), a gear floating state occurs in which a mutually pressing force is made weaker between gears of a certain gear pair on which the output torque of the electric motor for running is normally acted. In such a state, the transmission of vibrations due to engine rotation fluctuations (explosion fluctuations) to the meshing portion of the gear pair may cause tooth surfaces of meshing teeth to strike together while repeatedly colliding with and separating from each other in the meshing portion, thereby generating a gear bearing noise, i.e., so-called rattling noise. To reduce such a rattling noise, for example, in Patent Document 1, it is proposed that, if the output torque of the electric motor for running falls within a predetermined range and a condition of occurrence of the rattling noise is detected, an engine rotation speed is increased to a predetermined value or more by equal power while an engine torque is reduced to a predetermined value or less so as to suppress the engine torque fluctuations. Specifically, the engine is normally operated along a predetermined engine operation line (e.g., engine optimum fuel consumption line) allowing the engine to operate as efficiently as possible while the operating state of the engine (e.g., an engine operating point represented by an engine rotation speed and an engine torque) is smoothly changed in response to a necessary change in engine power. On the other hand, if the condition of occurrence of the rattling noise is detected, the engine rotation speed is increased to the predetermined value or more to shift the engine operating point from the engine optimum fuel consumption line to a predetermined engine operating line (e.g., rattling noise avoidance operation line) for avoiding the rattling noise on an equal power line.